Re: [Qemu-devel] [PATCH 1/2] rfifolock: add recursive FIFO lock

[Stefan Hajnoczi]

On Fri, Oct 11, 2013 at 04:55:31PM +0800, Wenchao Xia wrote:
> > +/* Recursive FIFO lock
> > + *
> > + * This lock provides more features than a plain mutex:
> > + *
> > + * 1. Fairness - enforces FIFO order.
> > + * 2. Nesting - can be taken recursively.
> > + * 3. Contention callback - optional, called when thread must wait.
> > + *
> > + * The recursive FIFO lock is heavyweight so prefer other synchronization
> > + * primitives if you do not need its features.
> > + */
> > +typedef struct {
> > +    QemuMutex lock;             /* protects all fields */
> > +
> > +    /* FIFO order */
> > +    unsigned int head;          /* active ticket number */
> > +    unsigned int tail;          /* waiting ticket number */
> > +    QemuCond cond;              /* used to wait for our ticket number */
> > +
> > +    /* Nesting */
> > +    QemuThread owner_thread;    /* thread that currently has ownership */
> > +    unsigned int nesting;       /* amount of nesting levels */
> > +
> > +    /* Contention callback */
> > +    void (*cb)(void *);         /* called when thread must wait, with 
> > ->lock
> > +                                 * held so it may not recursively 
> > lock/unlock
> > +                                 */
> > +    void *cb_opaque;
> > +} RFifoLock;
> > +
> If you respin, the define can be moved to util/rfifolock.c, leave
> typedef struct RFifoLock RFifoLock;
> in header.

Then the struct cannot be embedded as a field, it would require heap
allocation of all RFifoLocks.

This is why I chose to include the definition in the header file.

Stefan